In conventional color photography, photographic products contain three superimposed units of silver halide emulsion layers, one for forming a latent image corresponding to an exposure to blue light (blue-sensitive), one for forming a latent image corresponding to an exposure to green and one for forming a latent image corresponding to an exposure to red light.
During photographic treatment, the developing agent reduces the silver ions of each latent image. The resulting oxidized developing agent, then reacts in each unit with a dye-forming coupler in order to produce images in yellow, magenta and cyan dyes respectively from the recordings in blue, green and yellow. This produces negative dye images.
The reversible photographic products which make it possible to obtain positive images comprise the same three superimposed units of silver halide emulsion layers, each of these units containing respectively a yellow, magenta and cyan dye-forming coupler. After exposure, these reversible photographic products are subjected to a first black and white development (development of the latent image), and then to a step of chemical reversal or fogging exposure, which makes it possible to make the silver halides which were not initially exposed developable. After reversal, the photographic product is treated in a color development bath in the presence of couplers, generally contained in the photographic product.
Color photographic products are evaluated on the basis of sensitometric curves indicating the colored density of each of the yellow, magenta and cyan components as a function of luminance, that is to say the intensity of exposure. In order to achieve a good chromatic balance it is very important to obtain similar characteristic curves for the three sensitive layers, that is to say of the same form and superimposed. Indeed, when these curves are not similar, this causes a dominant or poor color rendition. When the characteristic curves of each of the sensitive layers are superimposed, an exposure of the photographic product in white light must give a neutral total density value, which corresponds to a neutral gray tone.
In order to reproduce detail in the image it is also important to use photographic products with a wide exposure latitude. The exposure latitude is a measurement of the suitability of a photographic product for recording the differences in exposure intensity and for representing them through differences in density. For a given range of exposure intensities, the more there are smaller differences in image density reproduced, the more details there are in the color image.
It is known that the exposure latitude of a photographic product can be increased by modifying the silver halide photographic emulsions. For example, it is known that the size dispersity of an emulsion can be increased in order to increase the rendition of details.
It is also known that a layer of silver halide emulsions can be chromatized over more than one region of the light spectrum in order to improve the reproduction of the colors of the image. For example, patent EP 304297 describes a photographic product comprising a layer of silver halide emulsions which is chromatized in two regions of the light spectrum in order to increase the exposure latitude.
U.S. Pat. No. 4,946,765 describes a color photographic paper which comprises a first and second layer of silver halide emulsion, each of these layers being sensitized in a different region of the light spectrum and containing a particular dye-forming coupler. In order to improve the exposure latitude of the product, this patent discloses the introduction into the product, between the two layers of emulsions, of an intermediate layer which is not sensitive to light and which contains a non-diffusible colorless coupler forming during development a complementary dye with the main sensitivity of the second silver halide emulsion layer.
All these modifications of the photographic product which tend to change its exposure latitude also change the form of the sensitometric curves of each of the light-sensitive layers and consequently impair the superimposability of the curves. By increasing the exposure latitude using these techniques, the chromatic balance is changed.